In general, an image sensor is a semiconductor device for converting an optical image into an electrical signal, and is roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
A CIS includes a photodiode and a metal oxide semiconductor (MOS) transistor in each unit pixel. To operate, the CIS sequentially detects electrical signals of the respective unit pixels in a switching mode, thereby realizing an image.
The related CIS includes a photodiode region that receives an optical signal to convert it into an electrical signal, and a transistor region that processes the electrical signal.
This related CIS has a structure in which the photodiode and the transistor are horizontally disposed.
Although the related CIS overcomes drawbacks of the CCD image sensor, the related CIS still has problems to be solved.
Specifically, according to the related CIS having the horizontal structure, the photodiode and the transistor are formed horizontally adjacent to each other on the substrate. Thus, the CIS requires an additional region for the photodiode on the substrate. Thereby, the CIS decreases a region corresponding to a fill factor, and restricts a possibility to increase resolution thereof.
Further, according to the related CIS having the horizontal structure, it is very difficult to optimize a process of simultaneously forming the photodiode and the transistor. In other words, the process of forming the transistor requires a shallow junction for low sheet resistance, but such a shallow junction is not suitable for the process of forming the photodiode.
Usually the related CIS having the horizontal structure additionally provides on-chip functions. Thus, a size of the unit pixel should be increased in order to maintain sensitivity of the CIS, or an area for the photodiode should be decreased in order to maintain a pixel size. However, if the pixel size is increased, the resolution of the CIS is decreased. Furthermore, if the area for the photodiode is decreased, the sensitivity of the CIS is decreased.